Function calls may be used to trigger processing circuitry to perform a corresponding function. There may be various competing sources of function calls, such as applications or interrupt handlers, and it may not be possible to handle all of the function calls right away. Therefore, a scheduler may be provided to schedule which function calls are processed when by the processing circuitry. Such schedulers often use a prioritization scheme in which each function call has an associated priority, and the scheduler determines which function calls should be processed first based on the priority of each function call. Some schedulers allow additional information to be defined for a function call. For example, it is possible to define a window of time in which the processing circuitry should process the function call, and the scheduler can then aim to schedule the function call so that it is processed within this window. However, the window information is regarded merely as a secondary hint for assisting with the primary scheduling mechanism based on the function priorities. While the window information may optionally be provided, not all function calls are provided with this information. The priority-based scheduling mechanism takes precedence.